1. Field of the Invention
The present invention relates to a reversible thermosensitive recording medium, more particularly to a reversible thermosensitive recording medium comprising a reversible thermosensitive recording layer, with the transparency or color thereof being reversibly changeable depending upon the temperature thereof, which is capable of recording information therein and erasing recording information therefrom by utilizing the reversibly changeable transparency or color of the reversible thermosensitive recording layer. The present invention also relates to a method of producing such a reversible thermosensitive recording medium.
2. Discussion of Background
Recently, reversible thermosensitive recording media, which are capable of temporarily forming images or recording information therein and also capable of deleting formed images or recorded information therefrom when such formed images or recorded information becomes unnecessary, have attracted attention.
Japanese Laid-Open Patent Applications 54-119377 and 55-154198 disclose representative examples of such a reversible thermosensitive recording medium, which comprises an organic low-molecular weight material such as a higher fatty acid, which is dispersed in a matrix resin such as a vinyl chloride--vinyl acetate copolymer having a low glass transition temperature (Tg) in the range of 50.degree.-60.degree. C. to less than 80.degree. C.
Such a reversible thermosensitive recording medium, however, has the shortcomings that the reversible thermosensitive recording layer thereof is deformed and the density and contrast of the formed images are lowered during repeated image formation and erasure by use of a heating element such as a thermal head.
In order to eliminate the above-mentioned shortcomings of the conventional reversible thermosensitive recording medium, and also in order to increase the durability of the reversible thermosensitive recording medium during repeated image formation and erasure thereof by use of a thermal head or the like, the inventors of the present invention have proposed in Japanese Laid-Open Patent Applications 5-169809 and 5-169810 that the average polymerization degree of a matrix resin for use in the reversible thermosensitive recording layer and the content of the vinyl chloride repeat unit contained therein be respectively limited to particular values, in particular, the average polymerization degree be increased to a particular value.
Furthermore, the inventors of the present invention have proposed to contain epoxy resin in the reversible thermosensitive recording layer and, in particular, to subject the reversible thermosensitive recording layer to thermosetting as disclosed in Japanese Laid-Open Patent Application 5-38872.
These proposals, however, have not achieved the desired effects sufficiently.
Furthermore, in Japanese Laid-Open Patent Application 5-085045, there is proposed a reversible thermosensitive recording medium comprising a reversible thermosensitive recording layer comprising as the matrix resin a thermosetting resin prepared from a hydroxyl-modified vinyl chloride--vinyl acetate copolymer and an isocyanate compound, in order to improve the heat resistance and mechanical strength of the reversible thermosensitive recording layer, thereby improving the repeated use durability of the reversible thermosensitive recording medium when a thermal head is used for image formation.
The thermosetting resin of the above-mentioned type, however, deteriorate with time with respect to the hardness thereof. More specifically, the hardness of the resin at the time of the formation of the reversible thermosensitive recording layer changes with time.
In particular, in the case of a reversible thermosensitive recording medium of the type in which an organic low-molecular-weight material is dispersed in a resin, the reversible thermosensitive recording layer thereof is usually transparent in a predetermined temperature range, and when the recording layer is heated to a temperature above the above-mentioned temperature range, the recording layer becomes milky white. Thus, image recording and image erasure are carried out in this reversible thermosensitive recording medium by utilizing the reversible changes from the transparent state to the milky white state and vice versa by selective heat application. When the above-mentioned reversible changes from the transparent state to the milky white state and vice versa are performed, it is preferable that the temperature range in which the recording layer maintains the transparent state stably (hereinafter referred to as the transparent temperature range) be broad to a certain extent.
However, in the case where the hardness of the resin employed in the reversible thermosensitive recording layer changes with time, the transparent temperature range is decreased with time, and it becomes impossible with time to erase images at the initially set erasure temperature. When this occurs, the setting of the erasure temperature becomes extremely complicated. In other words, the above-mentioned proposal has created the above-mentioned new problem. However, no proposal for solving this problem has been made yet.
Furthermore, recently the following problems have been reported with respect to the conventional reversible thermosensitive recording media:
Specifically, printing systems that perform printing on the conventional reversible thermosensitive recording media with the application of high printing energy thereto under the same conditions as those for a low-thermosensitive recording medium, for example, a thermal destruction type thermosensitive recording medium, is increasing in number. In this case, the energy for printing applied to the reversible thermosensitive recording media considerably exceeds the printing energy necessary for the formation of images on the reversible thermosensitive recording media, so that when thermal printing is performed on such reversible thermosensitive recording media by use of a printer for the above-mentioned thermal destruction type thermosensitive recording medium, the reversible thermosensitive recording media are caused to considerably deteriorate even by one printing operation, so that there is the tendency that sufficiently high image density and contrast for use in practice cannot be obtained thereafter.
There has not been proposed any countermeasure against the above-mentioned problems.